This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
In order to support mobility in a mobile network such as e.g. LTE (Long Term Evolution), a procedure referred to as HO (Handover) is employed whereby the mobile network can order the UE to connect to a different cell compared to the one to which it is currently connected. The reason for this is mainly mobility (as already mentioned) but could also be due to e.g. load balancing reasons.
To support the HO procedure, radio measurements are needed. A reporting criterion for these radio measurements are given by a set of HO parameters, which are defined in 3GPP standards with more details to be found in e.g. Sections 5.5.4 and 6.3.5 of 3GPP TS 36.331 “Radio Resource Control”, version: V12.4.1 (2014-12). For example, the HO parameters which typically control where the HO occurs between the cells may comprise TTT (Time To Trigger), CIO (Cell Individual Offset), Frequency Specific Offset, Event specific offsets (e.g. A2, A3, A4, A5, B1, B2), HO hysteresis etc.
If the set of HO parameters are poorly adjusted in such a way that a UE (User Equipment) does not report handover measurements on time, the UE might lose its connection with an original cell before the HO is successfully completed and before the UE managed to establish a connection to a target cell. Such cases will result in a HOF (Handover Failure). Avoiding HOF is one of critical challenges in RRM (Radio Resource Management).
FIG. 1 which consists of FIG. 1(a) and FIG. 1(b) shows a conventional complete X2-based intra-MME (Mobility Management Entity)/intra S-GW (Serving Gateway) HO procedure for an example LTE system, as described in detail in Section 10.1.2.1.1 of 3GPP TS 36.300 version: V12.4.0 (2014-12).
As seen from FIG. 1(a), the HO procedure includes three phases: HO Preparation, HO Execution and HO Completion.
Hereinafter, only the HO Preparation phase including steps 0 to 7 which is related to our invention will be discussed below.
Step 0: UE context within a source eNB 101 contains information regarding roaming and access restrictions which were provided either at connection establishment or at the last TA (Time Advance) update.
Step 1: The Source eNB 101 configures UE measurement procedures according to the information regarding roaming and access restrictions. Measurements provided by the source eNB 101 may assist a function controlling connection mobility of the UE 103.
Step 2: A Measurement Report is triggered and sent from the UE 103 to the source eNB 101.
Step 3: The source eNB 101 makes a decision based on the Measurement Report and RRM information to hand off the UE 103.
Step 4: The source eNB 101 issues a Handover Request message to the target eNB 105 passing necessary information to prepare a HO at the target side (UE X2 signaling context reference at source eNB, UE S1 EPC (Evolved Packet Core) signaling context reference, target cell ID, KeNB*, RRC (Radio Resource Control) context including the C-RNTI of the UE in the source eNB, AS-configuration, E-RAB context and physical layer ID of the source cell+short MAC-I for possible RLF recovery). UE X2/UE S1 signaling references enable the target eNB 105 to address the source eNB 101 and the EPC. The E-RAB context includes necessary RNL and TNL addressing information, and QoS profiles of the E-RABs.
Step 5: Admission Control may be performed by the target eNB 105 dependent on the received E-RAB QoS information to increase a likelihood of a successful HO, if the resources can be granted by target eNB 105. The target eNB 105 configures the required resources according to the received E-RAB QoS information and reserves a C-RNTI and optionally a RACH preamble. The AS-configuration to be used in the target cell can either be specified independently (i.e. an “establishment”) or as a delta compared to the AS-configuration used in the source cell (i.e. a “reconfiguration”).
Step 6: The target eNB 105 prepares HO with L1/L2 and sends a Handover Request Acknowledge message to the source eNB 101. The Handover Request Acknowledge message includes a transparent container to be sent to the UE 103 as an RRC message to perform the HO. The container includes a new C-RNTI, target eNB security algorithm identifiers for the selected security algorithms may include a dedicated RACH preamble, and possibly some other parameters i.e. access parameters, SIBs, etc. The HO Request Acknowledge message may also include RNL/TNL information for the forwarding tunnels, if necessary.
NOTE: As soon as the source eNB 101 receives the HO Request Acknowledge, or as soon as transmission of the HO command is initiated in the downlink, data forwarding may be initiated.
Step 7: The target eNB 105 generates a RRC message to perform the HO, i.e. RRCConnectionReconfiguration message including the mobilityControlInformation, to be sent by the source eNB 101 towards the UE 103. The source eNB 101 performs necessary integrity protection and ciphering of the message. The UE 103 receives the RRCConnectionReconfiguration message with necessary parameters (i.e. new C-RNTI, target eNB security algorithm identifiers, and optionally dedicated RACH preamble, target eNB SIBs, etc.) and is commanded by the source eNB 101 to perform the HO. The UE 103 does not need to delay the HO execution for delivering the HARQ/ARQ responses to the source eNB 101.
Generally, HO between cells can be subject to different kinds of problems, and natures of the problems can be disclosed by analyzing some message flow after the handover has been initiated. For example, the HO problems may comprise:                Too early HO problem, in which HO from a source cell (i.e. a source eNB) to a target cell (i.e. a target eNB) was initiated too early, and the UE failed to establish a new connection with the target cell (steps 9, 10 or 11 fails in FIG. 1(a)); the UE re-establishes the connection with the source cell; and thereby, the source cell is capable of determining that the HOF was due to a too early HO.        Too late HO problem, in which HO from a source cell to a target cell was initiated too late and the connection to the UE fails before a new one to the target cell has been initiated; the UE will re-establish in the target cell including information about its source cell and optional additional failure information (time of failure, radio conditions, etc.), and the target cell will inform the source cell about the failure, which enables the source cell to determine the type of HOF.        HO ping-pong problem, in which the UE returns to the source cell shortly after being handed over to the target cell; thereby, the HO is seen as unnecessary.        HO to WC (wrong cell) problem, in which the UE is handed over to a target cell, but shortly after the HO, the connection fails, and the UE re-establishes in a third cell; and the third cell informs the target cell about the failure, and forwards the information to the source cell to enable determination of the cause of the failure.        
In areas of a network where there is an overlap of more than two cells at a border, there is a risk that the UE is handed over to a WC (i.e., a non-intended cell), which could result in a HOF. Tuning the HO border based on this information could potentially affect the entire borders between the source cell and other neighboring cells.
FIG. 2 shows an exemplary scenario where a HO to WC problem occurs during movement of a UE.
In FIG. 2, the UE moving from CellA 201 via a path shown in a solid line could be handed over to CellC 205; but soon after the HO to CellC 205, the connection fails, and the CellC 205 should initiate another HO towards CellB 203. In this process, the HO to WC problem occurs. A similar HO to WC problem could occur on a path shown in a dotted line just by interchanging roles of the CellB 203 and the CellC 205 respectively.
The HO performance obtained by the conventional HO solution is reduced in a case that there is a risk that the HOF may occur due to the HO to WC problem e.g. when there is an overlap of more than two cells at the border which will be discussed in detail later.
Therefore, a HO solution capable of improving the HO performance in this case is desired.